Wave coil having a smoothed characteristic impedance



Sept. 29, 1942. H. J. R. VON BAEYER EI'AL 2,297,514

' WAVE COIL HAVING A SMOOTHED CHARACTERISTIC IMPEDANCE Filed Oct. 31, 1940 mentors Gttorneg Patented Sept. 29, 1 942 WAVE COIL HAVING A SMOOTHED CHAR- ACTERISTIC IMPEDANCE Hans Jakob Ritter von Baeyer, Hans Otto Roosenstein, and Robert Tantsen, Berlin, Germany; vested in the Alien Property Custodian Application October 31, 1940, Serial No. 363,666

a In Germany June 19, 1939 4 Claims.

In co-pending patent application, Serial No. 322,656 filed March 7, 1940, now Patent No. 2,258,261, issued Oct. 7, 1941, a coil having line characteristics has been disclosed in which the dependence of the line characteristic or property upon the frequency has been minimized by cross division of the dielectric or the Ierro-magnetic material. Although this step insures a substantial de-coupling between adjacentparts of the coil, the disturbing effect still remains'quite appreciable in the case of current distribution in which there is a marked dependence upon the distance in the direction of the coil axis. This applies particularly to the free end of the coil which experiences a sudden drop from a finite value to zero. This free end is graphically shown in Figure 1, while Figures 2, 2A and 3 illustrate the application of the present invention to overcome the problem,

Referring to Figure 1, I denotes the iron-core,- which, as disclosed in the earlier application, may be subdivided transversally, while the winding 2 terminates at the point marked A. The shield, denoted by 4, may be placed inside the coil as shown or outside iiydesired. Assuming,

for the sake of simplicity of explanation, that the coil is fed with afrequency so low that at points A, B, C practically the same current flows throughout, then it will be evident that a turn at B where there are neighboring turns both on the right and the left-hand'side, will have pothe potential of the turn in question arises practically onlyas a result of mutual inductance relation with the adjacent turns. But a turn at A, contradistinct to a turn at B, has neighbors only on the one side. Hence, th voltage induced therein is exactly one-half the induced voltage in .a turn at B, for the latter turns have twice as'many neighbors. Applied to a line and the considerations holdinggood therefor, this could also be expressed as if to a turn at A there inheres an apparent self-inductance only one-half to compensate for such deficiency, as it were, of inductance prevailing at the ends of the coil. To this end, two methods are suggested, both of them leading to the desired result.

In the first place, it is possible to compensate for such deficiency of inductance by providing an artificial increase in the inductance.

Furthermore, it ispossible to complete the deflciency of inductance by a reduction of the capacity in such a way that the surge or characteri'stic impedance remains stable throughout the length of the coil. In the'last mentioned solution, to be sure, the phase constant at the ends of the coil will turn out lower than in the middle. However, since it is the over-all phase constant that is important together with the fact that internal reflections due to changes in the characteristic impedance are avoided, an entirely satisfactory result is secured by this solution.

The change of the self-inductance as suggested in the first solution towards the free end. of the coil is accomplishable in a simple' manner by winding the coil more closely at the free ends. Moreover, as shown in Figure 2 wherein turns 3 are more closely spaced than the remainder, in the case of iron-cored coils the permeability of the iron or the quantity of iron taken per crosssection of the coil may be altered towards the ends either uniformly or discontinuously.

Figure 2A, for instance, shows an iron-core I! in schematic form which may be regarded, basically, as favorable for accomplishing the said end. Of course, it is also possible in the case of a transversally subdivided iron-core of the kind disclosed in the above-mentioned earlier application, to make the spacings between the various iron disks smaller in the direction towards the end of the coil. Finally, an increase in the inductance is obtainable by enlarging the diameter of the turns or of the core at the free end as compared with the diameter of turns in the middle of the coil. A few turns 2, 3 show this clearly. In fact, this step is useful also with air-cored coils.

The second solution above-mentioned may be carried into cheat in the following fashion: The specific capacity of the line per unit or length,

as pointed out, must b less at the free end than in the middle of the line.

Figure 3 shows the shielding 3d modified in accordance with the invention. As pointed out in the earlier application, the shield presents an axial split 5, the latter being here enlarged at 35 in the direction towards the end of the coil. The reduction of the capacity at the free end, of course, could also be insured by a gradual or stepwise increase in the thickness of the insulation material, that is to say, the distance der in the neighborhood of the winding and 'means for maintaining the ratio of inductance tocapacity per convolution of said coil constant throughout the length of said coil, said means including a longitudinal slit along the length of said cylinder, said slit expanding toward the ends.

' creased towards that end.

between this shield or a metallic cylinder arranged inside the coil and the turns of the latter. It is finally feasible also by reduction of the effective dielectric constant to reduce the capacity at the end of the coil.

whereby the ratio of inductance to capacity per convolution of the coil is maintained constant throughout the length of said coil.

2. A wave coil having a winding of a plurality of convolutions of a conductor, a shielding cylin- 3. A wave coil having a winding of a plurality of convolutions of a conductor and means assoelated with the convolutions at the ends of said coil for causing the ratio of inductance to capacity of the end convolutions of said coil to equal said ratio for the convolutions throughout the remainder of said coil.

4. A wave coil having a winding of a plurality of convolutions of a conductor and means associated with the convolutions at the ends of said coil for causing the ratio of inductance to capacity 'of the end convolutions of said coil to equal said ratio for the convolutions throughout the remainder of said coil, said means operating to increase the inductance of the end convolutions .of said coil.

HANS JAKOB BITTER VON BAEYER. HANS o'rro ROOSENSTEIN. ROBERT TANTZEN. 

